The present invention relates to a method of producing peroxide-cross-linked or polymerized polyethylene pipes in an extrusion line that includes an extruder, a cross-linking oven and a withdrawal mechanism or haul-off, wherein the cross-linking oven has a first heating section that extends from an inlet to an actuated deflection roller, and a second heating section that extends from the deflection roller to an outlet, with the extruded pipe being subjected to a stretching process. The present invention also relates to an extrusion line having a cross-linking oven comprised of a first heating section that extends from the inlet to an actuated deflection roller, and a second heating section that extends from the deflection roller to the outlet.
It is known that the properties of polymers can be improved by cross-linking or polymerization. In contrast to non-cross-linked pipes, peroxide-cross-linked polyethylene pipes have the advantage of enhanced heat resistance, mechanical strength and chemical resistance, which makes them suitable for under-floor heating pipes, hot water pipes and drinking water pipes.
DE 696 22 053 T2 describes an extrusion line for in-line cross-linking of polyethylene pipes. The extruder of this line has an upwardly angled head, which opens into a cross-linking oven, in which the pipe is subsequently extruded vertically. The pipe is led along a first heating section vertically upward in the cross-linking oven, reversed there by 180 degrees around an electric driven deflection roller and then led along a second heating section vertically downward, where it is deflected into the horizontal plane by another roller at the outlet of the cross-linking oven and then led through a calibration and cooling unit. On the way through the cross-linking oven, the pipe is led along infrared units, which provide the heat into the pipe that is required for cross-linking. The calibration and cooling unit is followed by a haul-off, with which the pipe is pulled through the extrusion line, and after which, it is finally coiled.
DE 10 2007 050 939 A1 describes a cross-linking oven, which has a first heating section that extends from its inlet to an electric driven deflection roller and a second heating section from the deflection roller to its outlet. In this case, a horizontally extruded pipe is at first deflected by 90 degrees around a roller at the inlet of the cross-linking oven, so as to facilitate its passage along the first heating section vertically upward in the cross-linking oven, reversed there by 180 degrees around a deflection roller and then led along a second heating section vertically downward in the cross-linking oven. The pipe is deflected back into the horizontal plane by a roller provided at the outlet of the cross-linking oven.
The extruded melt pipe swells up after its exit from the pipe head of the extruder. In order to produce a pipe with a required nominal diameter, the pipe must therefore be drawn later in the extrusion line. However, this stretching process has disadvantageous consequences for shrinkage, which is a key quality criterion for the final product. This shrinkage must be as small as possible.
Therefore, the object of the present invention is to provide a method and an extrusion line with a cross-linking oven, which guarantee minimum shrinkage of the pipe produced.